Titanium diboride is a known to be an extremely hard material, having a hardness typically in excess of 4.times.10.sup.3 Knoop Hardness Number (KHN) and is known to be a coating on refractory metal carbides.
Chemical vapor deposition to obtain titanium diboride has limited utility because of part configuration constraints. Different coefficients of thermal expansion from titanium diboride vapor deposited coating of most potential substrates limits the number and type of base materials severely, e.g. to tungsten carbide and graphite. Moreover, because chemical vapor deposition depends of fluid streams passing over the part surface being treated, the result is subject to discontinuities where flow patterns are undesirable, variations reflecting varying flow patterns, and withal an inability to develop an effective deposit in many areas of complex parts.
The commercial application of titanium diboride to refractory metal carbides has thus not been practically achievable. Nor therefore, have the benefits of this extremely hard, long wearing and erosion resistant composition been available on a wide range of parts, e.g. nozzles, valves, and pump components or on certain structurally superior materials, e.g. nickel, cobalt, chromium or iron matrix refractory metal carbide structures. Such coatings would be a major advance in the art of environment resistant equipment, and a highly significant breakthrough in such formed articles as choke valves in oil field equipment.
Recently, an attempt has been made to have the desirable properties of titanium diboride available on refractory metal carbides. The route chosen however, was chemical vapor deposition, with the result that in addition to all the problems inherent in vapor deposition, e.g. holidays, variable coverage, inability to cover complex shapes, extensive efforts and processing are required to attempt to hold the titanium diboride on the article surface, and prevent spalling. The problem is that forming titanium diboride at the article surface gives erosion and abrasion resistance no better than the adhesion of the coating to the substrate, regardless of the intrinsic hardness of the titanium diboride.
In U.S. Pat. No. 4,268,582 to Hale et al, tungsten carbide articles are subjected first to a chemical vapor deposition of boron, then a boron diffusion, followed by a chemical vapor co-deposition of boron and titanium to form a titanium diboride superstrate on the boron prediffused substrate, the substrate being expected to hold the superstrate on. Hale et al suggest that their result of a distinct superstrate coating of titanium diboride can be realized as well with molten salt bath deposition, pack diffusion and coating, and physical vapor deposition. However obtained, the coating approach to imparting the benefits of titanium diboride may be prone to the spalling failures which characterize all hard coatings not integrated with their surfaces, but only adhered thereto.